Reporting by Helen Hill  for MITgcm
A team from the UK has been using an ocean-only configuration of the MITgcm and its adjoint to explore the sensitivity of the Atlantic Meridional Overturning Circulation to surface heat and freshwater fluxes over the Subpolar Gyre.
The Atlantic meridional overturning circulation (AMOC) transports approximately 0.5 PW of heat northward across the Equator and impacts regional and global climate, with the circulation implicated as one of the possible factors that keep Western European winters mild compared to other parts of the Northern Hemisphere. The AMOC also plays an important role in the surface uptake and vertical distribution of heat and carbon within the ocean which in turn affects the evolution of surface climate under greenhouse gas and aerosol forcing.
Because of the AMOC’s outstanding importance for large-scale ocean and climate variability, a number of theoretical and modeling studies have explored the sensitivity of the circulation to surface buoyancy forcing at different latitudes, as well as the meridional transport connectivity in the Atlantic Ocean, ie, the extent to which AMOC anomalies are causally related across different latitudes. Here we report on a modeling study by Yavor Kostov , Helen Johnson , and David Marshall  from the University of Oxford in the UK who have been using MITgcm and its adjoint to probe the mechanisms behind the large seasonal differences in the AMOC sensitivity to surface heat they observe. Their findings are online in Climate Dynamics.
Using a 1 degree, ocean-only version of MITgcm, the researchers initialized the ocean state with realistic conditions based on the ECCO version 4 state estimate and drove the model with optimized time-evolving historical atmospheric forcing also available from ECCO. To generate the adjoint and compute the AMOC sensitivities to surface buoyancy fluxes linearized about the time-evolving model background state, the team used the proprietary algorithmic differentiation software TAF. Full details of forward model and adjoint are provided in the paper.
Kostov and his co-authors found that surface heat loss out of the Subpolar Gyre in the winter strengthens the AMOC at a lead time of approximately six months, but that the same surface heat flux anomaly in the summer leads to a delayed AMOC weakening that emerges at a lag of eight months. Probing further, the team found that in response to a summer surface cooling perturbation, the AMOC progressively weakened up to a lag of approximately 80 months, with the negative overturning anomaly then persisting for years.
“Compared with the sensitivity to surface heat fluxes, seasonality in the AMOC sensitivity to surface freshwater fluxes in our experiments was less pronounced,’ says Kostov, “there was also no sign reversal between the response to summer and winter perturbations.”
In particular, the team’s experiments highlighted the role of evaporation. “Heat flux anomalies over the Subpolar Gyre trigger changes in the rate of evaporation and hence affect the salinity of the mixed layer. Surface cooling gives rise to freshening in the following months, whereas warming leads to salinification. Persistent buoyancy changes due to salinity responses counteract the impact of heat fluxes to a varying extent depending on the seasonal mixed layer depth,” says Kostov. “On the other hand, air-sea feedback mechanisms exert a positive feedback on the AMOC response to surface freshwater flux perturbations both in the summer and in the winter months.”
To find out more about this work contact Yavor 
Sensitivity of the AMOC to evaporation minus precipitation [Sv per (kg m−2 s−1 sustained over 1 hour)] at various lead times in months. Red (blue) shading indicates that the AMOC strengthens in response to a hypothetical positive (negative) flux anomaly at the specified lead time. The sensitivities were smoothed using a diffusive Gaussian operator with a spatial scale of 3 grid points after Weaver and Courtier (2001) – animation credit: Y. Kostov 
This Month’s Featured Publication
- Yavor Kostov, Helen L. Johnson, David P. Marshall (2019), AMOC sensitivity to surface buoyancy fluxes: the role of air-sea feedback mechanisms , Climate Dynamics, doi: 10.1007/s00382-019-04802-4
Other New Publications this Month
Óscar Álvarez, Alfredo Izquierdo, Carlos J. González, Miguel Bruno, Rafael Mañanes (2019), Some considerations about non-hydrostatic vs. hydrostatic simulation of short-period internal waves. A case study: The Strait of Gibraltar , Continental Shelf Research, doi: 10.1016/j.csr.2019.05.016
Philipp Anhaus, Lars H. Smedsrud, Marius Årthun, and Fiammetta Straneo (2019), Sensitivity of submarine melting on North East Greenland towards ocean forcing , The Cryosphere Discuss., doi: 10.5194/tc-2019-35
Olivier Beaumont, Julien Herrmann, Guillaume Pallez, Alena Shilova (2019), Optimal Memory-aware Backpropagation of Deep Join Networks , [Research Report] RR-9273, Inria. 2019. ffhal-02131552f, https://hal.inria.fr/hal-02131552/document
Ludmila Carone, Robin Baeyens, Paul Molliere, Patrick Barth, Allona Vazan, Leen Decin, Paula Sarkis, Olivia Venot and Thomas Henning (2019), Equatorial anti-rotating day side wind flow in WASP-43b elicited by deep wind jets?  arXiv: 1904.13334 [astro-ph.EP]
Lars Czeschel and Carsten Eden (2019), Internal wave radiation through surface mixed layer turbulence , Journal of Physical Oceanography, doi: 10.1175/JPO-D-18-0214.1
Valeria Di Biagio, Gianpiero Cossarini, Stefano Salon, Paolo Lazzari, Stefano Querin, Gianmaria Sannino, Cosimo Solidoro (2019), Temporal scales of variability in the Mediterranean Sea ecosystem: Insight from a coupled model , Journal of Marine Systems, doi: 10.1016/j.jmarsys.2019.05.002
Feng Ding and Robin D. Wordsworth (2019) A New Line-by-line General Circulation Model for Simulations of Diverse Planetary Atmospheres: Initial Validation and Application to Exoplanet GJ 1132B , arXiv: 1905.04635 [astro-ph.EP]
Mohamad El Gharamti, Kevin Raeder, Jeffrey Anderson, Xuguang Wang (2019), Comparing Adaptive Prior and Posterior Inflation for Ensemble Filters Using an Atmospheric General Circulation Model , Monthly Weather Review, doi: 10.1175/MWR-D-18-0389.1
Yao Fu, Chunzai Wang, Peter Brandt, Richard J. Greatbatch (2019), Interannual variability of Antarctic Intermediate Water in the tropical North Atlantic , JGR Oceans, doi: 10.1029/2018JC014878
Guo, Daquan (2019), Numerical investigation of baroclinic tides in the Red Sea , Earth sciences and engineering dissertations, KAUST, doi: 10.25781/KAUST-5IT2Y
Mukesh Gupta, Carolina Gabarro, Antonio Turiel, Marcos Portabella, Justino Martinez (2019), On the retrieval of sea-ice thickness using SMOS polarization differences , Journal of Glaciology, doi: 10.1017/jog.2019.26
Nils Hutter and Martin Losch (2019), Feature-based comparison of sea-ice deformation in lead-resolving sea-ice simulations , The Cryosphere Discuss., doi: 10.5194/tc-2019-88
Jesús García Lafuente, Simone Sammartino, José C. Sánchez Garrido, Cristina Naranjo (2019), On the role of the bay of algeciras in the exchange across the strait of Gibraltar , Regional Studies in Marine Science, doi: 10.1016/j.rsma.2019.100620
Graham K. H. Lee, Jake Taylor, Simon L. Grimm, Jean-Loup Baudino, Ryan Garland, Patrick G. J. Irwin, and Kenneth Wood (2019), Exoplanetary Monte Carlo Radiative Transfer with Correlated-k I. Benchmarking Transit and Emission Observables , Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stz1418
Sachiko Mohanty, A.D. Rao, B. Yadidya (2019), Spatial and Temporal Variability of Semidiurnal Internal Tide Energetics in the Western Bay of Bengal , Pure and Applied Geophysics, doi: 10.1007/s00024-019-02221-4
G. Morello, C. Danielski, D. Dickens, P. Tremblin, and P.-O. Lagage (2019), An Independent Analysis of the Spitzer/IRAC Phase Curves of WASP43 b , The Astronomical Journal, doi: 10.3847/1538-3881/ab14e2/
Steven R. Ramp, J.-H. Park, Yiing Jang Yang, Frederick L. Bahr, and Chanhyung Jeon (2019), Latitudinal Structure of Solitons in the South China Sea , Journal of Physical Oceanography, doi: 10.1175/JPO-D-18-0071.1
Michael A. Spall, Robert S. Pickart, and Peigen Lin (2019), Frontogenesis and variability in Denmark Strait and its influence on overflow water , Journal of Physical Oceanography, doi: 10.1175/JPO-D-19-0053.1
Qiwei Sun, Yan Du, Yuhong Zhang, Ming Feng, Jasti S. Chowdary, Jianwei Chi, Shuang Qiu, Weidong Yu (2019), Evolution of Sea Surface Salinity Anomalies in the Southwestern Tropical Indian Ocean During 2010–2011 Influenced by a Negative IOD Event , JGR Oceans, doi: 10.1029/2018JC014580
Oscar Vergara, Rosemary Morrow, Isabelle Pujol, Gérald Dibarboure, Clément Ubelmann (2019), Revised global wavenumber spectra from recent altimeter observations , JGR Oceans, doi: 10.1029/2018JC014844
Do you have news about research using MITgcm? We are looking for contributions to these pages. If you have an interesting MITgcm project (ocean, atmosphere, sea-ice, physics, biology or otherwise) that you want to tell people about, get in touch. To make a post, contact Helen